70 PPM CO is definitely dangerous. Sleeping for 8 hours in 35 PPM may cause brain damage, along with false alarms for a device this sensitive.
The problem is CO has 300x the affinity for blood cells than does O2.
BTW, there is a slight vacuum inside of a moving car so if you have headaches, nausea or dizziness while driving look and listen for an exhaust leak.
Absolute *******s I have noted that you are from the glorious US of A but a standard condensing boiler registering 70PPM CO would not be unusual as long as the CO2 was within the Manufacturers recommendations